Low power wireless transceivers are used for a variety of applications such as cell phones, PDAs, tracking devices, etc. Such transceivers require the use of a duplexer and/or multiplexers to transmit and receive signals using a single antenna. To that end, the duplexer is coupled to an antenna, a receiver, and a transmitter. The duplexer passes signals having a first frequency received at the antenna to the receiver that processes such signals. Additionally, the transmitter sends transmit signals having a second frequency to the duplexer, which, in turn passes the transmit signals to the antenna for transmission. Many systems also include a digital baseband processor that processes inbound signals from the receiver and prepares outbound signals for transmission by the transmitter.
Some systems utilize transmitter signal refinement techniques that require a feedback loop to provide an indication of the signal being transmitted. For example, some systems utilize a feedback loop to facilitate the evaluation and correction of transmitter signal distortion. Such feedback loops are usually facilitated by the receiver. However, in full duplex operation mode, the receiver is completely utilized and, therefore, cannot be used to detect transmitter signal attributes that are useful in performing transmit transmitter signal refinement techniques (e.g., distortion control).
One solution to control and refine transmit signals (e.g., to control transmit signal distortion) has been calibration of the transmitter at the factory with calibration values stored in a memory used in normal operation. This approach is time intensive and increases the costs of development of transceiver designs. Additionally, this approach is not adaptive because the calibration values are stored in memory and are not typically changeable. Accordingly, the values used by the transceiver do not change with changing transceiver performance over time. In some situations, adjustments to precalibrated values are possible as a function of temperature, if the information about the temperature is available to the transmitter. In such a case, the temperature value is used to choose appropriate pre-distortion.
Another solution to controlling transmitter performance has been a feed-forward approach using computationally expensive methods such as look up tables or non-linear compensating functions. In another approach, transmitter feedback has been attempted by a power detector loop on the transmitter. However, a power detector loop does not contain any signal phase data and, thus, provides incomplete signal information for feedback and control purposes. In such a case, Cartesian feedback is applied with I/Q down-conversion, a loop filter, and a feedback inside the PA module. However, it is difficult to design such a system to house good stability and noise performance. Moreover, because such an approach is an analog approach, this approach is very susceptible to voltage, temperature and process variations. For example, the PA can pull the supply voltage as it is ramped, which complicates the design of these analog feedback circuits. Secondly, the noise introduced by the feedback path can make the use of this approach very limiting for cellular standards. Preamplifiers have also been used to attempt to refine the transmitter signals. However, the use of a preamplifer is limited due to noise and bandwidth issues from the transmitter.